1. Field of the Invention
The present invention relates to a single-cylinder or in-line multi-cylinder engine and a straddle-type vehicle provided with the same.
2. Technical Background
For instance, JP-A-2004-249867 discloses a motorcycle in which an engine is mounted in a manner that a cylinder thereof is inclined rearward. JP-A-2004-249867 discloses that three merits can be obtained by rearward inclination of the cylinder. The first merit is that load distribution on a front wheel can be increased. The second merit is that the body weight can be reduced. The third merit is that traction can be improved with extension of a rear swing arm.
There is a demand for further improved motion performance in a straddle-type vehicle such as a motorcycle. As a method of improving the motion performance, concentration of mass in the engine can be raised, for example. More specifically, there are at least three heavyweight objects in the engine: a cylinder head, a crankshaft, and a gear change mechanism. A method of disposing these three heavyweight objects as close together as possible can be applied. The application of the method causes the cylinder head to be located between the crankshaft and the gear change mechanism. In this way, distances between the center of gravity of the vehicle and the centers of gravity of the cylinder head, the crankshaft, and the gear change mechanism become shorter, and mass is concentrated. As a result, the motion performance can be further improved.
The rearward inclination of the cylinder, as in the motorcycle disclosed in JP-A-2004-249867, can be considered as playing a part in disposing the cylinder head, the crankshaft, and the gear change mechanism in proximity to each other for concentration of mass. However, it is difficult to concentrate the mass merely with the rearward inclination of the cylinder to the gear change mechanism side. Positional interference occurs between the cylinder body and the gear change mechanism when the cylinder is inclined rearward. Therefore, the gear change mechanism has to be disposed away from an axis of the crankshaft in order to incline the cylinder rearward. Because of this, it is difficult to concentrate the mass merely with the rearward inclination of the cylinder. Also, it is difficult to incline the cylinder rearward to a degree that is enough to gain an effect of concentration of mass, without extension of a distance between the gear change mechanism and the axis of the crankshaft.